Diesel fuel for engines in cars and trucks, for example, usually contains a material that tends to precipitate out as a solid wax at lower ambient temperatures. This wax restricts flow, clogs fuel filters and fuel injectors, and makes starting and running difficult. It is therefore desirable to heat the fuel sufficiently to dissolve the wax and keep it in solution before the fuel is filtered and injected. Heaters for this purpose have been developed and are in use. One such heater is described in U.S. Pat. No. 4,424,422, assigned to the same assignee as the present invention. Heat is applied to the fuel as it flows through the fuel line by an electrical resistance heater when the temperature of the fuel drops below a predetermined temperature at which wax precipitation occurs. The heater is automatically turned off when the ambient temperature of the fuel rises above such temperature.